It is the long range goal of this projjct to determine whether tumor promotion by various agents, e.g., phorbol esters, proceeds via the release of arachidonic acid (AA) and the subsequent synthesis of prostaglandins (PG). We have examined several fibroblasts and chosen the mouse embryo fibroblast (10T 1/2) as a model system. PG production by these cells was examined with respect to cell growth, and cell density. The kind of PG produced by 10T 1/2 was dependent on the number of passages. Phorbal esters (TPA) altered cell growth and PG production. If a correlation can be determined between PG release and tumor promotion, we will have established a basis for the development of a test system for screening environmental tumor promoters.